Generally, X-ray detectors are basically pin (p (p type doping), i (intrinsic), and n (n type doping)) structures in a semiconductor. The n 102 and p 106 type electrodes are large in area having homogeneous contacts on the front and back 310 sides. See FIG. 1 (standard planar detector 100) and FIG. 3 (Second generation silicon drift detector (with integrated FET 300)). Detectors that operate in room temperature environments have resolutions normally limited by two factors (1) impurity concentration and (2) device capacitance. The impurity concentration is determined by the quality of the starting material and the cleanliness of the semiconductor processing. The capacitance is determined by the electrode design. In the case of large homogenous contacts, such as N+ ohmic bulk contact (OV) 102, the capacitance is inherently large.
Thick silicon drift detectors (SDDs) have better “X-ray stopping power” due to the increased silicon mass. Denser semiconductor materials such as Cadmium telluride (CdTe) and Cadmium zinc telluride (CdZnTe) have also increased X-ray stopping power in comparison to standard (thin) silicon SDDs. The advantage of the present invention over CdTe or CdZnTe based detectors is the result of a much lower noise level due to very low capacitances.
“Silicon drift detectors” have set new records as room-temperature semiconductor detectors for X- and gamma-ray spectroscopy. When compared with classical photomultiplier tubes (PMTs), SDDs offer the typical advantages of a silicon photo-detector, i.e., a higher quantum efficiency for scintillation light, the possibility of a monolithic integration of arrays of photo-detectors of almost any area and shape, and the immunity to magnetic fields. Moreover, if compared with conventional silicon pn-photo-detectors of the same active area and thickness, SDDs allow for better energy resolutions and lower detection thresholds due to the lower level of electronics noise arising from the smaller value of output capacitance.
Most SDDs have device thicknesses around 300 μm and applied voltages at the drifting cathodes 1302 range from −200 to −30 V (see FIG. 13A and 13B). A 300 μm device-thickness limits the practical x-ray detection efficiency to about 20 keV. The x-ray efficiency for 300 μm thick Si falls off rapidly above 10 keV, with 50% efficiency at 15 keV and only 9% at 30 keV. Currently, there are a large number of XRF (X-ray fluorescence) measurements that can benefit from the increase in efficiency at higher energies afforded by these new thick devices, including DoD, DoE, medical, space, and Homeland Security applications.
The general concept of a silicon drift detector (SDD) was first proposed by E. Gatti, P. F. Rehak, Nucl. Jnstrum. Methods 225, 608 (1984) and later realized by P. Rehak, J. Walton, E. Gatti, A. Longoni, M. Sampietro, J. Kemmer, H. Dietl, P. Holl, R. Klanner, G. Lutz, A. Wylie and H. Becker, Nucl. Instrum. Methods A 248, 367 (1986). In an SDD (referring to FIG. 2) two superimposed fields from the device front- and back-side direct generated electric carriers electrons 108 to a small collecting anode 206. The general electronic noise performance is improved in comparison to standard x-ray detectors because of the small size of the collecting anode 206 (low capacitance). FIG. 2 shows a first generation SDD, where drift strips 202 on the back (see back plate 210)- and front-side generate a lateral “drift-field” (see path 208) for the carrier.
Detectors with high-aspect ratio holes (or trenches) were first introduced by S. Parker, C. Kenney and J. Segal, Nucl Inst. Meths. A 395, 328 (1997) as radiation hard detectors, having vertical electrodes to deplete laterally the detector matrix—see FIG. 6A and FIG. 6B; these holes (see hole array 602) were fabricated by micro-machining techniques.
Several improvements to the SDD have been published, see e.g. L. Strüder, presentation at Advanced Instrumentation Seminars, SLAC, Stanford, [Presentation conducted Feb. 7, 2007, accessed on the Internet on Sep. 24, 2009 at: [http://www-group.slac.stanford.edu/ais/past SeminarDetails.asp?seminarID=60]. FIG. 3 shows a second generation silicon drift detector with an integrated Field Effect Transistor (FET) 300. The FET300, having a source 302, gate 304 and drain 306, acts as an amplifier and yields to higher resolution. FIG. 4 shows an SDD in a “bulls-eye” shape with an integrated FET 400.
Contemporary SDDs incorporate planar geometry. The different generations of SDDs vary as to how the drift electrodes are arranged on the front- and back 310-surface. Most SDDs have device thicknesses around 300 μm and applied voltages at the drifting cathodes 1302 range from −200 to −30 V (see FIG. 13A and 13B); see for example: P. Lechner, S. Eckbauer, R. Hartmann, S. Krisch, D. Hauff, R. Richter, H. Soltau, L. Strueder, C. Fiorini, E. Gatti, A. Longoni and M. Sampietro, Nucl. Instrum. Methods A 377, 346 (1996); and A. Castoldi, C. Guazzoni, E. Gatti, A. Longoni, P. Rehak, L. Strueder, “IEEE Trans. Nucl. Sci. NS 44, 1724 (1997); and C. Piemonte, A. Rashevsky, A. Vacchi, Microelectronic J, 37, 1629 (2006).
A 300 μm device-thickness limits the practical X-ray detection efficiency to about 20 keV, see FIG. 5. The x-ray efficiency for 300 μm thick Si falls off rapidly above 15 keV, with 50% efficiency at 15 keV and only 9% at 30 keV. Currently, there are a large number of XRF (X-ray fluorescence) experiments that could benefit from the increase in efficiency at higher energies afforded by thicker devices.
So far, only C. R. Tull et al. presented up to 1.5 mm thick SDD detectors, see C. R. Tull, J. S. Iwanczyk, B. E. Patt, S. Barkan, L. Feng, IEEE Trans. Nucl. Sci. NS 51, 1803 (2004). Very high resistivity float zone (FZ) material (26,000 Ωcm resistivity) was used as a substrate in order to minimize the required operating voltages. The guard ring structure was designed to hold up 1,000 V bias. The device size varied from 10 to 20 mm2.
Therefore, the need exists for a fabrication method to produce thicker silicon drift detectors with improved hard x ray spectroscopy performance over current room temperature semiconductor detectors.